1. Field of the Invention
The present invention relates to a method for surface treating a heat exchanger, a surface treatment agent, and an aluminum heat exchanger.
2. Related Art
The heat exchanger portions of an air-conditioner and the heat exchanger portions of an air purification system are made in a complex structure in which aluminum fins are retained between aluminum tubes in narrow intervals in order to improve heat-transfer efficiency. For this reason, the surface of the aluminum fins are chemical conversion treated as necessary and then hydrophilized, whereby the shedding of moisture condensed during air conditioning is facilitated (e.g., refer to Japanese Unexamined Patent Application Publication No. H05-302042).
The fins and the like of the heat exchanger are often assembled by brazing an aluminum alloy. Methods of brazing can be separated into methods using and not using flux. Although the vacuum brazing method (VB method) can be exemplified as a method not using flux, and the non-corrosive flux brazing method (NB method) using a fluoride based flux can be exemplified as a method using flux, due to excelling in corrosion resistance and being low in facility cost, there has been a trend of shifting to the NB method. However, since the NB method causes the flux dissolved in water and in a slurry state to adhere to the heat exchanger, there has been a problem in that flux remains after brazing and a foul odor emanates from this.
Although it is necessary to remove residual flux in order to solve such a problem, it is difficult to completely remove the residual flux by pickling being conventionally performed as a step prior to chemical conversion treatment. For example, Japanese Unexamined Patent Application Publication No. H11-131254 discloses a method for surface treating an aluminum-containing metal material in which a chemical etching treatment is conducted with an acidic aqueous solution containing at least one selected from the group consisting of sulfuric acid, hydrofluoric acid, nitric acid and phosphoric acid prior to hydrophilizing treatment, and performing chemical conversion treatment using an aqueous solution of zirconium phosphate or titanium phosphate thereafter; however, the effect on removing deposits of residual flux and the like has not been adequate in such a method for surface treatment either. In addition, concealing the residual flux has been difficult with chemical conversion treatment after pickling.
As another method for surface treating a heat exchanger, a method has been disclosed in which a chemical conversion coating film is formed, and then a second protective layer is formed thereon composed of a hydrophilic, poorly water-soluble resin coating film including: (a) a resin component having at least one hydrophilic group selected from a primary, secondary or tertiary amino group, a quaternary ammonium group, an amide group, a carboxyl group, a sulfo group, an ethyleneoxide group, a phosphonate group, and a hydroxyl group, and a cross-linking reactive group of a different kind than the above-mentioned hydrophilic group selected, and selected from an amide group, carboxyl group and hydroxyl group; and (b) a cross-linking agent component containing (i) a trivalent-chromium compound and (ii) a fluoro complex salt in a total amount exceeding the cross-linking equivalent amount of the resin component (e.g., refer to Japanese Unexamined Patent Application Publication No. 2001-174192). However, this method was not targeted at heat exchangers prepared by the non-corrosive flux brazing method.
In addition, a method is disclosed in Japanese Unexamined Patent Application Publication No. 2002-30462 in which pickling is performed using ferric salt, a chemical conversion treatment is performed after deposits of the brazing filler material have been effectively removed, and then a hydrophilizing treatment is performed. However, the chemical conversion treatment agent used in such a method is a chromate-based chemical conversion treatment agent, which is not desirable to use, mainly from the viewpoint of the burden on the environment.
Treatment agents that make a heavy metal such a zirconium, titanium, and hafnium as the coating film forming component can be exemplified as non-chromium chemical conversion treatment agents. However, with the chemical conversion coating film obtained using the aforementioned such non-chromium chemical conversion treatment agents, such as a zirconium-based chemical conversion treatment agent in particular, there is a problem in that a uniform chemical conversion coating film cannot be formed on the brazed heat exchanger, and thus the adherence with the hydrophilic coating film obtained using a conventional hydrophilizing treatment agent is not sufficient. As a result, degradation of the hydrophilic coating film advances from repeated cooling and heating over an extended period of time, and the chemical conversion coating film also degrades accompanying the degradation of the hydrophilic coating film. The odor characteristic to the flux remaining on the surface of the heat exchanger becomes a problem due to the hydrophilic coating film and the chemical conversion coating film degrading and the surface of the heat exchanger being exposed in this way.
In particular, although there is a description in Japanese Unexamined Patent Application Publication No. 2002-30462 with regard to an odor suppressant composed of an organic substance having an amide group and/or phenol group, such an odor suppressant is used in order to suppress the odor characteristic to the chromate contained in a chromate-based chemical conversion treatment agent, and does not provide any kind of solution to the suppression of odor, characteristic to a case of performing chemical conversion treatment using a non-chromium chemical conversion treatment agent, resulting from the degradation of the hydrophilic coating film and chemical conversion coating film.